Processes heretofore employed for the preparation of cyclic phosphonitrilic chloride oligomers usually produce a significant proportion of higher cyclic materials and linear materials which are undesirable for the ring-opening polymerization process. Eliminating these less desirable higher cyclic and linear phosphonitrilic chloride species requires extensive purification procedures and also reduces productivity.
The prior art teaches several general approaches toward the desired goal through control of the reaction. For example, high dilution of the reactants appears to favor increased cyclic content, the use of an excess of finely divided ammonium chloride particles favors increased cyclic content, and the slow and even addition of one reactant to the other appears to favor cyclic content. These general approaches have been described for example in U.S. Pat. Nos. 3,367,750, 3,667,922, 3,656,916, 3,658,487 and 3,780,162. Nevertheless, these prior processes produce significant amounts of petroleum ether insoluble linear phosphonitrilic chloride oligomers which must be separated from the desired cyclic species.
U.S. Pat. No. 4,196,179 to Hudson and Dominick teaches the preparation of cyclic phosphonitrilic halide oligomer mixtures by simultaneous reaction of PCl.sub.3, Cl.sub.2 and NH.sub.3 in an inert solvent at 65.degree.-180.degree. C. The amount of higher cyclic (i.e., (PNCl.sub.2).sub.n where n is 5 or above) and linear oligomers in the product is generally in the range of 11 to 28 percent.
Various kinds of catalysts have been investigated to boost the yield of cyclic oligomers and particularly cyclic trimer. Jenkin and Lanoux (1970) investigated the effect of anhydrous metal chlorides on the yield of cyclic trimer from the reaction of NH.sub.4 Cl and PCl.sub.5 in a very dilute solution. Their results show that a few metal chlorides (e.g., MnCl.sub.2, MoCl.sub.5) increase trimer yield (total trimer yield 35-50%), but other metal chlorides (e.g., CoCl.sub.2, CuCl.sub.2, HgCl.sub.2, LaCl.sub.3, NiCl.sub.2, PbCl.sub.2, etc.) reduce the trimer yield (11-30%). They also show that the reaction rate is increased by the addition of the metal halide but this rate increase does not necessarily mean a trimer yield increase.
U.S. Pat. No. 4,248,845 (to Otsuka Chemical Company) teaches the preparation of tri- and tetra-phosphonitrilic chlorides in a shortened period of time in high yield by reacting NH.sub.4 Cl with PCl.sub.5 in an inert solvent in the presence of a selected metal oxide as a catalyst. However, the results given in this patent show that this process produces 14 to 29% of higher cyclic oligomers and linear material (1 to 23% of linear materials) depending on the nature of the metal oxide. This patent also indicates that without the use of the catalyst the yield of higher cyclic and linear oligomers will be in the range of about 58 to 60%.
More recently, U.S. Pat. No. 4,382,914 (to Nippon Fine Chemical Company) teaches the preparation of cyclic (PNCl.sub.2).sub.n oligomers in high yield by reacting PCl.sub.5 with an excess of NH.sub.4 Cl in an inert solvent in the presence of a metal chloride or an organic acid salt as a catalyst. Their results show that without a catalyst, 28.5% of linear phosphonitrilic chloride materials are obtained under the same reaction conditions. Although the process of the patent produces cyclic oligomer mixtures in high yield by using a catalyst, it apparently does not produce cyclic trimer in high yield. Yields of trimer ranging from 56 to 75% are shown.